1. Field of the Invention
The present invention relates to a sliding surface discharge preionization device, and method for gas discharge lasers, and particularly to a high capacitance sliding surface discharge that uniformly illuminates the discharge area and strongly preionizes the laser gas therein.
2. Discussion of the Related Art
Excimer lasers for industrial applications such as microlithography, TFT annealing, micromachining or flat panel display manufacturing, among others, must provide an output beam with high energy stability. Energy stability improvements in excimer lasers have been made by using laser beam parameter feedback algorithms for computer-controlling the laser gas composition and the driving discharge voltage (see U.S. patent applications Ser. Nos. 09/379,034, 09/447,882, 09/418,052, 09/484,818, 60/137,907, 60/160,126, 60/159,525, 60/171,717, each of which is assigned to the same assignee as the present application, and U.S. Pat. Nos. 6,014,398, 6,005,879, 5,710,787, 5,463,650 and 6,008,497, all of which are hereby incorporated by reference). Improvements in discharge chamber design have improved the flow uniformity of the laser gas through the discharge area further improving the energy stability (see U.S. patent application Ser. No. 09/453,670, assigned to the same assignee as the present application and hereby incorporated by reference).
The energy stability of the excimer laser is also strongly influenced by the strength and uniformity of the preionization of the laser gas within the discharge volume. The xe2x80x9cpreionizationxe2x80x9d of the laser gas corresponds to the initial electron concentration in the discharge volume at the initial stage of the discharge sequence. There have been developed several preionization devices and methods for generating short wavelength UV radiation that interacts with the laser gas in the discharge volume. Among the various preionization techniques are sliding surface discharge (see DE 29521572 and U.S. Pat. Nos. 5,081,638 and 5,875,207, each of which is hereby incorporated by reference), corona discharge (see U.S. patent applications Ser. Nos. 09/247,887 and 60/162,845, and U.S. Pat. No. 5,247,531, each of which is assigned to the same assignee as the present application, and DE 3035730, 3313811, 2932781, and 2050490, all of which are hereby incorporated by reference), and spark discharge between a number of pin electrodes (preionization gap) either in series arrangement (see U.S. Pat. No. 4,105,952, hereby incorporated by reference) or in parallel (see U.S. Pat. No. 4,287,483, hereby incorporated by reference) to the main discharge (see also U.S. Pat. Nos. 4,980,894, 4,951,295, 4,797,888, 5,347,532, each of which is assigned to the same assignee and is hereby incorporated by reference).
The sliding surface discharge according to DE 29521572 and U.S. Pat. No. 5,875,207 is an efficient and promising method for the preionization of the excimer laser gas medium. It is a type of discharge at the surface of a dielectric medium. The surface discharge guarantees radiation in the UV and VUV spectral range down to a wavelength of xcex=2 nm at a plasma temperature in the discharge of up to 3xc3x97104xc2x0 K. (see also Bagen B., Arbeitsbr. Ins. Plasma Phys., Julisch 1963, pp. 631-34, hereby incorporated by reference).
The ""638 patent, mentioned above and illustrated at FIG. 1a, describes a sliding surface discharge preionization arrangement 1a wherein insulating material 2a is positioned between preionization pins 4a to bridge the gap between the pins 4a. The insulating material 2a provides a xe2x80x9ctracking surfacexe2x80x9d 6a for a preionization discharge. Similarly, the ""638 patent, mentioned above and illustrated at FIG. 1a, describes a sliding surface discharge preionization arrangement 1b wherein insulating material 2b is positioned between preionization pins 4b to bridge the gap between the pins 4b. The insulating material 2b provides a xe2x80x9ctracking surfacexe2x80x9d 6b for a preionization discharge.
The main advantage of the arrangement according to the ""638 patent is the minimization of wear on the electrode pins 4a of FIG. 1a and the pins 4b of FIG. 1b, which is typically a problem with conventional spark gap preionizer arrangements. The voltage needed to drive a sliding surface discharge is less than that needed for dielectric breakdown of the gas between the pins 4a of FIG. 1a and the pins 4b of FIG. 1b. An additional advantage is that significant output laser parameters are more stable for excimer and molecular fluorine lasers having sliding surface discharge preionizers than those having spark preionizers. For example, the use of sliding surface discharge pre-ionization arrangements typically provides better pulse-to-pulse energy stabilities as well as a longer dynamic gas lifetimes for excimer and molecular fluorine lasers .
A disadvantage of the arrangements shown at FIGS. 1a and 1b of the ""638 patent is the directional characteristics of the UV light generated by the sliding discharge preionization. While spark-type pre-ionization arrangements with pin electrodes produce characteristically cylindrical radiation patterns, the UV emission from the sliding surface discharge preionization arrangements of FIGS. 1a and 1b is directed away from the insulating tracker surface. This results in a periodic preionization intensity distribution along the elongated direction of the discharge volume.
In addition, the sparks pins 4a of FIG. 1a and the pins 4b of FIG. 1b of the preionization arrangements are described in the ""638 patent as being located nearer the high voltage main electrode than the grounded main electrode (the main electrodes are not shown in FIGS. 1a and 1b), the UV emission from the tracker surface 6a of FIG. 1a and the tracker surface 6b of FIG. 1b more strongly illuminates the portion of the laser gas in the discharge volume that is nearer the high voltage electrode than the grounded electrode. Thus, the laser gas in the discharge volume is not uniformly illuminated. In addition, due to the pin-type structure of the pins 4a of FIG. 1a and the contact shape of the electrodes 4b of FIG. 1b, the energy coupling into the surface discharge is somewhat inefficient.
It is therefore an object of the invention to provide an efficient sliding surface preionization arrangement for an excimer or molecular fluorine laser.
It is further object of the invention to provide a sliding surface preionization arrangement wherein the laser gas filling the discharge volume of the excimer or molecular fluorine laser is uniformly illuminated with UV light generated by the sliding surface discharge.
In accordance with the above objects, an excimer or molecular fluorine laser is provided with a sliding surface discharge preionization arrangement for efficiently generating UV light that uniformly illuminates the laser gas located within the discharge volume. The preionization arrangement includes one or more preionization units including an elongated electrode and a plurality of pin electrodes, wherein an elongated insulating dielectric is disposed between the elongated electrode and each of the pin electrodes. The elongated electrode and the pin electrodes preferably contact opposed surfaces of the dielectric, such as may be defined as xe2x80x9ctopxe2x80x9d and xe2x80x9cbottomxe2x80x9d surfaces of the elongated dielectric.
At least a portion of the insulating dielectric includes a sliding discharge surface at a long axis, or xe2x80x9csidexe2x80x9d, edge of its cross-section substantially facing the discharge volume of the laser. A portion of each of the elongated preionization electrode and the pin electrodes conductively contacts one of the cross-sectional short axis, or top and bottom, surfaces of the dielectric portion.
Preferably, the portion of the elongated electrode that contacts the dielectric is also a short-axis surface of the electrode. In addition, a significant area of the surface of at least one of the electrodes contacts the corresponding surface of the dielectric such that preferably the contact area is substantially larger than the area of the sliding discharge surface. Moreover, the sliding surface is preferably oriented and positioned at a location with respect to the discharge volume to direct the UV light at the discharge volume such as to optimize the uniformity of illumination of the laser gas therein.
The preionization arrangement of the present invention thus has an advantageous elongated dielectric including a surface for generating a sliding discharge, and strong capacitive coupling, such as around 0.1 pF or more, of the conducting electrodes to the insulating material creating high preionization of the laser gas in the discharge volume. Multiple sliding surfaces of the preionization arrangement of the present invention advantageously extend between the elongated electrode and each of the pin electrodes and are designed to irradiate efficiently the volume of the main discharge of the excimer or molecular fluorine laser.